Highly successful exploration and production of oil and gas in recent years from deepwater Gulf of Mexico (GOM) and other parts of the world have generated high interest in leasing, exploring and developing deepwater petroleum resources. Deepwater presents many new challenges with the design of long slender structures such as marine risers in which the length of the structure is significantly greater than the other two dimensions. Marine rises are especially susceptible to the potentially damaging effects of dynamic disturbances such as Vortex Induced Vibrations (VIV). Risers used in offshore drilling and oil and gas production provide a critical safety function as well as protecting the environment from oil spills. These long slender tubular structural elements must operate reliably in the harsh marine environment, sometimes for decades. As the water depth of exploration and production operations has increased, so has the challenge imposed on the design and reliability of risers and other long slender structures subjected to vibrations induced by water and wind induced loads. Corrosion or damage can occur and VIV can impose excessive stress and fatigue damage. The capability to inspect and monitor long slender structure dynamic performance has therefore become an important technology issue; particularly when the inspection must be conducted in situ.
Although steel has been the material of choice for risers for decades, composite and titanium risers are increasingly gaining attention because of the weight and economic advantages they provide in deepwater. The composites and oil industry have supported research and development on composite risers for over twenty years and although many believe the technology is ready for deployment, the expectation has not yet been fulfilled except in isolated trial deployments. Discussion of issues associated with the introduction of composite risers into service can be found in the following references, which are incorporated herein by reference: Douglas B. Johnson, Donald D. Baldwin, and K. Him Lo: “Composite Production Riser Development and Qualification Test Results,” Composites for Offshore Operations-3, S. S. Wang, J. G. Williams and K. H. Lo, Eds. University of Houston-CEAC, 2001, pp. 109-123 and by J. Murali, M. M. Salama, O. Jahnsen, and T. Meland: “Composite Drilling Riser-Qualification Testing and Field Demonstration,” Composite Materials for Offshore Operations-2, S. S. Wang, J. G. Williams, and K. H. Lo, Eds., American Bureau of Shipping, 1999, pp. 115-128. Probably the most important factor limiting the acceptability of this needed service is the reluctance of regulatory agencies to approve introduction of such safety critical component without the accompaniment of reliable methods for inspection. The oil and composites industries recognize the need for proven inspection methods to accompany into offshore service new safety critical applications such as composite risers. Likewise there is a need to monitor the safety of large diameter ropes used for station keeping of offshore platforms and of large cables commonly used in civil engineering structures subjected to water or wind induced vibrations. Discussion of issues associated with Vortex Induced Vibrations in this type of long slender structure can be found in the following reference, which is incorporated herein by reference: J. Kim Vandiver and O. M. Griffin: “Measurements of the Vortex Excited Strumming Vibrations of Marine Cables,” Ocean Structural Dynamics Symposium, Corvallis, Oreg. September 1982.
Vortex Induced Vibrations (VIV) is a very important design constraint imposed in the design of deepwater risers and one must address the effect it can have on the fatigue life. Discussion of some of the design issues associated with VIV in risers can be found in the following references, which are incorporated herein by reference: S.S. Wang, X. Lu, and T.P. Yu: “Vortex Induced Vibrations (VIV) of Composite Production Risers,” Composites for Offshore Operations-3, S. S. Wang, J. G. Williams and K. H. Lo, Eds. University of Houston-CEAC, 2001, pp. 199-213 and by J. Kim Vandiver: “Research Challenges in the Vortex-induced Vibration Prediction of Marine Risers”, Proceedings of the Offshore Technology Conference, Paper No. 8698, Houston, May 1998. Although metal components have established methods of inspection when there is ready access, in situ methods are not well established or available. Composites exhibit different failure modes than metals and have different physical characteristics and specialized inspection methods such as ultrasonic, radiography, and acoustic emission have been used; however, they do not address the need to make deepwater in situ measurements. The availability of in situ Non-Destructive Evaluation (NDE) monitoring techniques for high performance safety critical long slender structures such as risers is urgently needed in the petroleum industry to accelerate the acceptance of composites technology and to address safety and reliability concerns for both metal and composite components. Information concerning the vibration characteristics and maximum bending strains being imposed on risers would permit intervention using active avoidance methods or help to improve design solutions such as described in the following reference, which is incorporated herein by reference: J. Kim Vandiver, K. Vikestad and C. M. Laren: “Norwegian Deepwater Riser and Mooring: Damping of Vortex-Induced Vibrations,” Proceedings of the 2000 Offshore Technology Conference, Paper No. 11998, Houston, Tex., May 1-4, 2000. U.S. Pat. No. 6,401,646 to Masters, et al. describe the application of shrouds, strakes and fairings to reduce the dynamic effects of Vortex Induced Vibrations on pipes immersed in a fluid, which is incorporated herein by reference.
One very effective way of monitoring structural performance is to measure the strain response to load. Strain can be compared to design predictions and monitoring the change in strain during service can be a very effective indicator of structural degradation due to overload, impact, environmental degradation or other factors. Advanced fiber optics technology is a reliable in situ method not only to measure peak strain values but bending strain information can be used to determine the vibration response imposed during dynamic loading such as by Gulf of Mexico loop currents. Bending strain is represented by the difference in the strain along the longitudinal axis measured at opposite ends of an imaginary line drawn perpendicular to the longitudinal axis of the long slender structure and through the structure centroid. The maximum bending strain occurs on an axis perpendicular to the structure longitudinal axis and perpendicular to the axis of zero bending strain. Fiber optics technology including Optical Time Domain Reflectometry (OTDR), Optical Frequency Domain Reflectometry and Bragg defraction grating methods are ideally suited for in situ measurement of strain in long slender structures. Bragg gratings are particularly valuable for making local strain measurements while the Optical Time Domain Reflectometry method is ideally suited for making global strain measurements such as the average strain over the length of a riser or several risers.
Fiber optics technology has matured rapidly in recent years with emphasis for use both in communications and for structural monitoring. U.S. Pat. No. 6,550,342 to Croteau, et al. disclose a non-intrusive method for measurement of the flow characteristics of a fluid in a pipeline based on an apparatus for varying the gain (sensitivity) of an fiber optic sensor using a circumferential strain attenuator. U.S. Pat. No. 6,271,766 to Didden, et al. discloses a fiber optics sensing system focused on the measurement of pressure, temperature, liquid fraction, flow, acoustic, seismic, resistivity, corrosion, and pipe wall build-up. The system records the data in a manner which allows selective billing only for the specific measurement services provided. U.S. Pat. No. 5,649,035 to Zimmerman, et al. discloses a fiber optics strain gage patch which measures the local strain response of a structure like a bridge. The strain gage is constructed of circumferential loops of an optical fiber to increase the strain measurement sensitivity and uses OTDR instrumentation to measure the strain.
OTDR is a time of flight method which measures spatial positions along an optical fiber by launching brief pulses of laser light into one end of the fiber and then detecting the subsequent reflections at reflective interfaces inserted along the length of the fiber. The principles of the use of optical fibers technology to measure strain are well established. Discussion of OTDR principles can be found in the following reference, which is incorporated herein by reference: M. K. Barnoski, M. D. Rourke, S. M. Jensen, and R. T. Melville: “Optical Time Domain Reflectometer,” Applied Optics, Vol. 16, No. 9. September 1977. The optical fiber is rigidly attached to the long slender structure and thus experiences strain identical to that imposed on the structure. By measuring the transit time of the reflected pulses and by knowing the speed at which light travels in the optical fiber, a very accurate measure of the distance to each reflective interface can be obtained. As the gauge section defined as the length between two reflective interfaces placed within the optical fiber undergoes strain, the interface's spatial position along the fiber changes and the OTDR measurement of this change in length is a direct measurement of the average strain in the structural component. An OTDR with a picosecond pulsed light source can measure a change in length as small as 0.4-inch with an accuracy of about +−.0.001 inch. A change in length of 0.4 in a 70-ft riser converts to a strain of 0.05%.+−.0.001%, which is sufficiently accurate to measure strains in the expected range of 0.07%. If needed, the accuracy can be increased in the riser application by making more than one traverse loop along the length of the pipe and thus provide a longer gage length. A single optical fiber can be used to measure strains at more than one location by imposing multiple reflective surfaces along the length of the optical fiber in combination with customized software algorithms to measure strain between each adjacent reflective interface. Measurement of the longitudinal strain in a long slender structure provides valuable information about the state of the “fitness for service” when compared to design allowables and expected conditions.
Although OTDR and Bragg diffraction are the preferred methods for making strain measurements, other optical fiber methods can also be used including Optical Frequency Domain Reflectometry (OFDR). The primary use of OFDR is for measuring reflections in optical fiber networks. In an OFDR instrument, the optical frequency of the signal laser is modulated in a periodic manner. The OFDR instrument uses an internal reflection to provide a reference of the modulated output. Light reflected from interfaces in the fiber returns to the ORDR, and these light signals are mixed with the reference signal, producing a mixture of optical frequencies. The waveform resulting from this frequency mixture is then analyzed using Fast-Fourier Transfer (FFT) signal analysis. This analysis provides information on the spatial positions of the each reflectance. The primary advantages of the OFDR technique are the excellent signal-to-noise and dynamic range characteristics.
Vortex-induced dynamic motion imposed by ocean currents typically have a period greater than 2 seconds and a wavelength involving several lengths of riser. Both the OTDR and Bragg diffraction grating techniques can be used to measure the bending strains imposed by VIV on offshore marine risers. With the OTDR method, the measured bending strains will have a value which can be interpreted to calculate the average radius of curvature. Although monitoring a single riser at a critical location may be sufficient, several risers segments at selected locations along the entire riser string can also be monitored including the region adjacent to the ocean floor. By placing optical fibers sensors on diametrically opposite sides of the tube, one can determine the strains due to bending which occur during the dynamic vibration imposed by the ocean currents, i.e., VIV. Since the direction of bending is not know, several diametrically opposed optical fiber sets must be introduced into the composite tube to be assured of obtaining the maximum bending effect. With sufficient numbers of bending strain measurements, one can also determine the location of nodes (locations along the length without bending) and thus the periodic wave length. From maximum bending strains one can calculate the radius of curvature and thus the vibration amplitude. The vibration frequency can also be determined since the strain measurements are made as a function of time.
Many methods have been developed to inspect the integrity of metal and composite components including ultrasonic, radiography, and acoustic emission. Each of these methods, however, does not address the need to inspect the component while performing the intended function in the marine environment. An in situ method is needed to monitor in real time the strains experienced by an offshore component such as a metal or composite riser. The fiber optics method of the current invention provides the capability to determine the state of fitness for service of these composite tubular components and provide the data needed to enhance safety and thus provide important operational safety quality assurance.
Visual inspection using a Remotely Operated Vehicle (ROV) is another method employed in offshore operations to inspect risers but a visual method is only superficial and does not provide the strain data needed to make precise engineering assessment of the structural integrity of the riser.
An Optical Time Domain Reflectometry (OTDR) technique for measuring the strain in a mooring rope is described in U.S. Pat. No. 6,999,641 B2 in which plastic optical fibers are used to make direct measurement of the large axial strains typically experienced by offshore platform mooring ropes (3% and higher). Detailed discussion of the application of plastic optical fibers to the measurement of strain in large diameter ropes is presented in the following references which are incorporated herein by reference: Smith, D. Barton and Williams, Jerry G.: Monitoring Axial Strain in Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers. 22nd International Conference on Offshore Mechanics and Arctic Engineering, Cancun, Mexico. Jun. 8-13, 2003. and by Jerry G. Williams and D. Barton Smith: “Direct Measurement of Axial Strain in Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers,” Fourth International Conference On Composite Materials for Offshore Operations. Houston, Tex., Oct. 4-6, 2005. The strains in risers are significantly lower (less than 1%) and thus allow glass optical fibers to be used which exhibit much less attenuation (loss of light) than plastic optical fibers yet have a practical strain limitation well within the range of the riser and similar long slender structures. The application of fiber optics to the riser application is significantly different from the rope application, however, the OTDR hardware required to make strain measurement is generally applicable and is similar to that used in the telecommunications industry to measure the location of broken optical fibers.
The deficiency of the visual inspection method is that it reveals nothing about the strain in the riser or provide adequate information to assess the criticality of Vortex Induced Vibration and most likely; for operational safety concerns, would not be available during the most critical load history of the riser such as during storms. In addition, it is difficult and expensive to reliably inspect risers in situ using ROV technology.
Traditional nondestructive inspection methods do not address the need to inspect and make measurements in situ in the marine environment.
Another long slender structure to which the strain measurement technology described above applies is to characterize dynamic behavior of large diameter ropes and cables subjected to water or wind generated vibrations. Large denotes a rope or cable sufficiently large in cross-section that bending strains are detectable as the long slender structure experiences vibration induced local bending. Large diameter mooring ropes, for example, are used for station keeping of offshore platforms and are subjected to VIV similar to that described above for risers. Large cables used as guide wires to stabilize tall towers is an example of a long slender structure subjected to wind generated vibrations.
A further application for the measurement of bending strains using the optical fiber system in long slender structures described herein is to characterize the buckling behavior of metal or composite spoolable pipe as it is injected into a small diameter annulus such as an oil well bore hole. Discussion of composite spoolable pipe technology can be found in the following reference: Jerry G. Williams and Alex Sas-Jaworsky II: “Composite Spoolable Pipe Development, Advancements, and Limitations,” Offshore Technology Conference Paper 12029. Houston, Tex., May 1-4, 2000. Spoolable pipe used as coiled tubing is typically taken off a large spool on which it has previously been wound and injected into a bore hole by the counter-rotating head of a coiled tubing injection rig. The counter-rotating heads of the Coiled Tubing Injector grip the pipe and impose axial compressive force to push the coiled tubing into the bore hole. Advanced applications of coiled tubing involve deployment into deviated and horizontal wells as well as vertical wells. Significant resistance to insertion can build up at the insert end of the coiled tubing requiring greater and greater injection force to keep it moving into the bore hole. As the resistance increases a condition is reached in which the applied compression force becomes sufficient to cause the pipe to buckle into a Spiral or Helical Buckle pattern (similar to a 3-dimensional “S” shape) inside the annulus. As large and larger axial compression forces are applied, the wave length (axial length of a repeated buckle pattern) of the spiral buckle gets shorter and shorter and a condition develops in which the spoolable pipe becomes locked in the annulus and cannot be withdrawn by the coiled tubing injector as it applies axial tension to retract the long slender structure.
Many long slender structures such as risers are composed on multiple discrete length segments joined together by threaded couplings to form the structure. In the coupling transition region between jointed segments the optical fiber is typically channeled into a coupling box attached to each end of the jointed segment. The optical fiber leads going into the coupling box are protected against bending and damage by the bonding agent and by the outer protective layer. The optical fiber leads are isolated against strain in the transition region by inserting them in a protective tube. Reflective interfaces may be placed at any location along the assembly including within a single segment or between multiple segments. For reflective interfaces located in different segments, the gage length is adjusted to account for the zero strain segment in the vicinity of the end termination and fiber optics connection box. Optical fiber signals are transmitted to the electronic optical signal monitoring instrumentation by means of an optical fiber or alternatively, the electronic optical signal monitoring instrumentation can be located remotely and digitized data transmitted by electric signal hard wire or radio signal to the accessible data acquisition system.
It is therefore an object of the invention to describe an optical fiber strain measurement system incorporating glass optical fibers or large strain plastic optical fibers to determine the bending strain in long slender structures and to use the bending strain information for the purpose of determining the structures vibration or buckling characteristics. In the application, the optical fibers are integrally attached near the outside of a metal or composite long slender structure using a bonding agent such as epoxy and protected from the environment including sea water and service damage by the bonding agent and an additional layer of polymer or elastomeric material and thus experience identical strain to that imposed on the structure.
An alternative location for placement of optical fibers for metal spoolable pipe used as coiled tubing is to place them on the inside of the pipe. Positioning the longitudinal optical fibers on the inside of the pipe addresses the potential problem of damage to the optical fibers by the gripper blocks of a coiled tubing injector. For composite spoolable pipe, the optical fibers can be positioned anywhere in the cross-section if inserted during the manufacturing process. One process for attaching the optical fibers on the inside of long slender tubular is to prefabricate a cylindrical metal or composite foil containing an adhesive, such as a high temperature epoxy, on the outer surface of the foil and to locate longitudinal optical fibers on the foil at selected locations around the circumference of the foil. After fabrication of the metal coiled tubing, the foil would be pulled inside the tubing and the foil cylinder internally pressurized with a hot fluid or gas to force the foil to expand and to cure the bonding agent to the inside of the long slender tubular structure.
It is another object of the invention to provide a strain measurement system for assessing the vibration characteristics and structural integrity of large ropes and cables using optical fiber strain measurement methods to determine the bending strain at selected locations along the long large rope or cable.
It is another object of the invention to use time of flight optical fiber strain measurement methods including Optical Time Domain Reflectometry (OTDR) to determine the bending strains in long slender structures by placing optical fibers along the axis of the metal or composite long slender structure starting at one end and traversing to the other end and if needed, to provide greater strain resolution; to loop the optical fiber back and forth as many times as needed to amplify the displacement magnitude.
It is another object of the invention to provide a method using the Optical Time Domain Reflectometry (OTDR) fiber optics method to measure bending strains in a long slender metal or composite structure made up of jointed segments including a single or multiple long segments including the entire length.
It is another object of the invention to provide a system utilizing Optical Time Domain Reflectometry (OTDR) and axially oriented optical fibers to determine the magnitude of bending strains imposed on metal or composite tubulars subjected to cyclic vibration such as induced in a metal or composite riser subjected to ocean currents, i.e., Vortex Induced Vibrations (VIV).
It is another object of the invention to provide a fiber optics system to measure the large axial bending strains typically imposed on long slender metal or composite spoolable pipe during deployment from a spool and into and out of a small diameter annulus using plastic optical fiber composed of polymeric materials including polymethyl methacrylate and perfluorocarbon, which have strain capabilities exceeding 5-percent with relatively low attenuation. It is another object of the invention to provide a fiber optics method to measure the bending strains imposed on spoolable composite pipe during deployment into and out of a small diameter annulus for the purpose of preventing a condition of lock-up of the pipe inside the annulus.
It is another object of the invention to provide a means to incorporate multiple fiber optics bending strain sensors within a long slender structure or segment thereof.
It is therefore an object of the invention to provide a capability through multiplexing of the Optical Time Domain Reflectometry (OTDR) and Bragg Diffraction instrumentation to allow numerous optical fibers or multiple reflections within the same fiber to be monitored using a single or multiple instruments.